1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same.
2. Related Art
In order to reduce the parasitic resistance of word lines of a flash memory device, a tungsten silicide film is used for the word lines.
However, the word line width of a flash memory device has been diminished along with the recent miniaturization of semiconductor devices. Consequently, it has become increasingly difficult to sufficiently reduce the parasitic resistance of word lines by still utilizing the resistivity (approximately 50 μΩ·cm) of the tungsten silicide film.
Hence, there has been proposed a method of using a cobalt silicide film, titanium silicide film (approximately 20 μΩ·cm), or the like, the resistivity of which is lower than that of the tungsten silicide film for word lines. These silicide films can be formed using a salicide process. The term “salicide” refers to a silicide made in a self-aligned manner. A salicide can be formed by forming a film of metal on word lines (polysilicon), forming a silicide by applying heat treatment to cause the metal to react with silicon, and removing excess metal using a chemical solution.
Prior to forming a cobalt silicide film on word lines using a salicide process, an insulating film, such as a silicon dioxide film, is buried between word lines using a chemical vapor deposition (CVD) method. When burying a silicon dioxide film in a thin, deep trench, such as one between word lines, using a CVD method, the surfaces of portions of the silicon dioxide film being deposited on the side surfaces of a word line come into contact with each other in the course of film forming. After such contact, a reactant gas for CVD is no longer supplied. Consequently, a joint line (seam) is formed in the center of the silicon dioxide film buried between word lines. Before a cobalt silicide film is formed, a natural oxide film present on the surfaces of word lines (polysilicon) is removed by etching treatment using dilute fluorinated acid or the like. At that time, the abovementioned seam, which is vulnerable to etching using dilute fluorinated acid or the like, expands in a groove-like manner.
After the formation of the cobalt silicide film, a silicon nitride film is deposited on the entire surface of a substrate. Between word lines in the silicon dioxide film, there is a trench formed by etching using dilute fluorinated acid or the like, and the silicon nitride film having a large dielectric constant is buried in this trench. Consequently, the proposed method has had the problem that capacitance between word lines increases, thereby causing the signal writing/reading speed of a flash memory device to decrease.
The proposed method also has had the problem that when a silicide film is used for a control gate, capacitance increases for reasons of, for example, wiring lines being too close to each other due to a reduction in the distance therebetween caused by miniaturization.